Multi-valued linear resistors are required for applications in integrated circuits, including for example, for static random access memory (SRAM) circuits, monolithic filters, and fusible links for programmable read only memories or redundant circuits. In a known method of providing a linear resistor for a MOS, Bipolar, or Bipolar CMOS (BiCMOS) silicon integrated circuit, a layer of polysilicon is deposited over a thick layer of dielectric, for example a field oxide provided on a silicon substrate. The polysilicon layer is patterned to define a resistor structure and then selectively doped by ion implantation. Typically, the polysilicon resistor structure comprises heavily doped end regions forming contact electrodes, and a resistive region extending between the contact regions which is doped sufficiently to provide a desired resistance value.
The latter method of fabricating resistors is applicable to forming either n- or p- type ohmic resistors, particularly low and medium valued resistors for applications in MOS and bipolar digital and analog ICs. Resistors in the low to medium resistance value ranges (i.e. 10-1000 ohm/sq) are linear over a wide voltage range. High value (gigaohm) leakage current resistors, used for example in applications for high density static random access memories (SRAMS), are more difficult to fabricate reproducibly, because undoped or lightly doped polysilicon is required to obtain high resistivities. The latter is more susceptible to process induced damage and defects, which degrade quality, create leakage and lead to poor reliability.
More significantly, for a process to provide multi-valued linear resistors on an integrated circuit, the resistive portion of each resistor of a different value requires a separate implantation step, with a different dopant dose, to provide a different doping level. Thus multiple photoengraving steps are required, and the number of masks depends on the number of different values of resistors required.